According to real patient reviews on RealSelf, the price can range from just $3,650 to $11,500
Cost varies by geographic location, surgeon and implant chosen (not the size of the implant; saline more expensive than round silicone and contour silicone more than round silicone). My advice is to not save a few dollars by having an unqulaified surgeon do your surgery and/or having your surgery in a non-accredited facility. Such can put your safety as well as the quality of your result. Cheap work is not so inexpensive if it has to be redone. Make sure your surgeon is a member of the American Society of Plastic Surgeons - plasticsurgery.org will give you a list of such qualified surgeons in your area.
In my practice, saline would be $2500.00 and silicone gel $3500.00 total. The size (unless greater than 1000cc) does not affect the price.